[r=Andy Doan, PS Jenkins bot] Remove hardcoded values for the series and archive from the branch source builder and allow specification of the archive (master ppa) as an environment variable. 1285820 from Francis Ginther

These workers weren't acking their rabbit message when upstart restarted
the job. We need to ack the message with a failure response so that
the task won't get run when we start back up. Otherwise, you can
have a job stuck in an infinite loop.
1285401 from Andy Doan